This invention relates to a process for economically recovering chromic acid from a bichromate solution. In particular, it relates to a process in which a bichromate solution is partially converted in an electrolytic cell at a conversion below that typically required for selective crystallization through evaporation and/or crystallization.
At the present, there are two principal commercial manufacturing methods for converting sodium bichromate into chromic acid. In the electrolytic method, an aqueous solution of sodium bichromate is electrolyzed. After a certain amount of the sodium bichromate is converted into chromic acid, the recovery of CrO.sub.3 from the solution is possible through evaporative crystallization or evaporation followed by cooling. This method has the advantage of not producing byproducts that must be recycled or disposed of, but, because of the high acidity of the solution which results from the degree of electrochemical conversion required in order to recover CrO.sub.3 selectively, it uses large amounts of electricity and is very expensive. See Canadian Pat. No. 739,447 and U.S. Pat. No. 4,273,628, herein incorporated by reference.
In the sulfuric acid method of producing chromic acid from sodium bichromate, an aqueous solution of sodium bichromate is mixed with sulfuric acid, forming chromic acid, which precipitates, with or without evaporation, before or after adding sulfuric acid, and sodium bisulfate. While the sulfuric acid method is less expensive than the electrolytic method, it generates a sodium bisulfate stream which must be recycled back to the chromate process or disposed of. See U.S. Pat. No. 4,290,864, herein incorporated by reference.